Stannate immersion coating for magnesium, magnesium-dissimilar metal couples, and other metals



United States Patent STANNATE IMMERSKON COATING FOR MAGNE- SIUM, MAGNESIUM-DISSIMILAR METAL COU- PLES, AND OTHER METALS Herbert K. De Long and Charles W. Glesner, Midland, and James A. Brown, Essexville, Mich, assignors to The Dow Chemical Company, Midland, Mich., a corporation of Delaware No Drawing. Filed Aug. 7, 1961, Ser. No. 129,505

24 Claims. (Ql. 1tl61) This invention relates to the electroless surface treatment, by immersion, of magnesium, magnesium base alloys, magnesium-dissimilar metal assemblies, and metals other than magnesium. The surface treatment involves the plating of tin or the coating of a combination of tin and MgSn(OH) on the metal treated. Hereafter, magnesium will be used to refer also to magnesium base alloys.

The ditficulty in surface treatment of magnesium products is well recognized. Many methods have been tried. None, however, have successfully attacked the problem of immersion treatment without the use of a strike coat. In the general methods of treatment, assemblies of magnesium and other metals must be treated separately before assembly. Attempts to treat the parts after assembly have not been successful. In assemblies of magnesium and other metals, the magnesium is sacrificed tothe other metal in a corrosive medium. This means that the magneisum has had to be protected from the dissimilar metal to prevent its becoming anodic and its subsequent destruction.

No prior successful work has involved the immersion coating of magnesium and magnesium-dissimilar metal assemblies. Since the two major purposes of surface treatment are to improve appearance and corrosion resistance, it is necessary that any coating be uniform, adehre well, and have good corrosion resistance qualities.

It is, accordingly, the principal object of the invention to provide a method for the coating of composite articles of magnesium and other metals, magnesium alone, and

other metals with a uniform, adherent, corrosion resistant coating, which has good appearance and good paint retention qualities, and to provide compositions useful in practicing such method.

The invention, whereby said object is achieved, may be stated broadly as being a method wherein a cleaned magnesium article, with or without exposed dissimilar metal components, or an article of a metal other than magnesium, is subjected optionally to an activator pickle treatment, and is finally coated in an immersion coating bath containing as essential dissolved constituents, a Water-soluble stannate and a water-soluble pyrophosphate. Other modifiers may be present, including, usually, enough caustic alkali to provide a pH in the bath near 13.2. This coating process is an electroless immersion process. The immersion coating operation is usually followed with a neutralization treatment, especially if the surface is to be painted.

For the treatment involved in this invention, it is desirable that the surface to be treated be cleaned. This may be done either mechanically, or chemically, or both, demnding on the nature of the foreign material on the surface. Another factor in determining what method of cleaning to use is the desired surface texture. If a very "Ice smooth texture is desired, a rough mechanical means would not be appropriate. Any conventional metal degreasing and cleaning method may be used.

After cleaning, the surface of the metal may be activated by a rinse in an activator pickle, such as the phosphoric acid-ammonium acid fluoride treatment used to prepare magnesium for zinc immersion coating. It is possible to eliminate this activating pickle step by adding a relatively small amount of a water-soluble acetate e.g. 0.25 to 3 percent sodium acetate to the coating bath.

This invention involves the composition of an immersion coating bath suitable for coating magnesium surfaces and for plating other metals, such as iron, steel, brass, or copper, alone or in assemblies of magnesium and dissimilar metals. This invention also includes methods of use of the immersion coating bath referred to above.

The bath may be prepared by first dissolving the watersoluble stannate e.g. potassium stannate (K SnO -3H O) in water at room temperature, to provide, for example, 37 to 60 grams per liter, usually followed by the addition of a caustic alkali e.g. sodium hydroxide (NaOH). The solution is then heated to 60-85 C. and the watersoluble pyrophosphate e.g. tetrasodium pyrophosphate (Na P O is added to provide 30 to grams per liter. The solution is stirred until the pyrophosphate is dissolved and the bath becomes clear. An acetate such as sodium acetate (NaC I-I O -3H O) may then also be added in amount to provide 7 to 30 grams per liter.

The caustic alkali is added to raise the pH of the solution to the desired level (about 13.2). This higher pH results in a more uniform, gray tin-containing coating on the metal being plated. Where no caustic alkali is added, the coating is black or dark gray. Samples run at the lower pH are useful, but they showed a streaked, uneven coating. Addition of caustic alkali sufficient to raise the pH much above the recommended value of 13.2 was found to retard the reaction, with a resultant lack of coating on the magnesium surface. The pH of the bath with no caustic alkali added is about 12, and with the caustic added in the desired amount, the pH is raised to about 13.2.

The pH of the bath should be adjusted periodically during use by the addition of caustic alkali, because the coating operation results in a lowering of the pH. Typically, the pH will decline from 13.2 to 12.8 during plating of 1720 square centimeters of magnesium surface per liter of coating bath.

The acetate is added to the bath in order to complex the precipitates formed in the reaction, mainly MgSn(OH) This helps to keep the solution clear and gives a longer bath life. Another reward from this addition is a more uniform coating, with a resultant increase in corrosion resistance. The better coating is a result of the complexing of the reaction products. This prevents the MgSn(OH) from being precipitated on the magnesium surface by a sedimentation process which would leave a loose coat having poor adherence and appearance. The sodium acetate also acts as a surface activating agent eliminating the need for the activating pickle mentioned above.

The bath may be operated without the acetate and without the caustic alkali, but for best results in coating magnesium surfaces, the solution composition should be as recommended below in Table I.

Table 1 As substitute for the above constituents in the coating bath, sodium stannate is a useful equivalent to potassium stannate, potassium hydroxide may replace sodium hydroxide, and sodium acetate may be replaced by either ammonium acetate or potassium acetate. The ranges of concentrations for these substitute compounds are the same as the recommended ranges in the preferred bath composition given above in Table I.

If the concentrations of the stannate and pyrophosphate salts are less than 45 grams per liter, a useful but nonuniform, gray to dark gray, streaked coating will form on magnesium surfaces. Concentrations greater than 55 grams per liter produce a lighter, uneven but still useful coating.

The bath is operable at temperatures ranging from room temperature to the boiling point of the solution. In general operation, however, the bath temperature is maintained at about 8085 C. This speeds the reaction so that an immersion time of about minutes is sufficient. At room temperature, a period of from 8 to 16 hours is required.

As previously stated, it is possible to use a bath of the stannate and pyrophosphate without the addition of the sodium hydroxide or sodium acetate. The concentrations of stannate and pyrophosphate in a bath of this sort are somewhat higher than those recommended above in Table I. This solution contains from 45 to 60 grams per liter of potassium stannate and from 45 to 100 grams per liter of sodium pyrophosphate. Use of this solution produces a dark gray to black coating on magnesium, This coat greatly reduces the general and galvanic corrosion of the coated metal, but is not as desirable as the' first recommended coating bath from an appearance and adhesion standpoint. This solution also has a shorter bath life due to the MgSn(OI-I) precipitate formed during use. This solution will operate at a higher rate if heated to the temperature recommended for the first solution, but it too is operable at ranges from room temperature to the solution boiling point if the immersion time is suitably adjusted.

A major advantage of thisinvention is that not only magnesium but magnesium-dissimilar metal assemblies may be coated in one operation. I-Ieretofore, this has been unheard of without a great loss of corrosion resistance.

The corrosion resistance of magnesiumdissimilar couples coated as described in this invention has proved to be excellent in comparison with prior methods of protection.

Both of the types of solutions described in this invention will plate a combination of about 50 percent Sn and 50 percent BgSn(OH) on a magnesium surface. The other metal surface receives a true tin plate.

This feature makes these solutions excellent for use in a barrel plating operation where the metal'to bebarrel plating, the barrel is lined or stripped with the anodic material which is contacted with the metal to be plated by the tumbling motion of the barrel. The anodic material may be magnesium or any other suitable metal electropositive to the metal to be coated, but below sodium in the series. Immersion time would be dependent on the electromotive potential which may limit the choice of anodic metal. It is to be noted that if a metal other than magnesium is to be plated, it must V sive electrical anodizing apparatus.

be contacted with magnesium or a metal electropositive to the metal to be coated.

After treatment of a metal surface in one of the baths described above, the surface should be neutralized if it is to receive further treatment with organic materials at this stage (e.g. painting). This is due to the high alkalinity of the coating. This neutralization may be done by immersion in a 5 percent ammonium acid fluoride for 2 minutes, or in a 5 percent sodium bifluoride for 1 minute, or in some comparable solution for an appropriate time. Either neutralization suggested here may be carried out at room temperature.

Comparison with other recommended treatments for magnesium in various exposure tests show that this method of coating is at least equal to the previous best known methods for corrosion protection. The comparisons set forth below were between the Dow #17 anodize process, as described in US. Patent No. 2,901,409, and the immersion process and bath described in this invention. Galvanic corrosion of dissimilar metal couples was greatly retarded in both painted and unpainted systems. Adhesion characteristics were also good, being equal to other paint bases recommended for magnesium.

A major advantage of this invention over the Dow #17 anodize process is that it is not necessary to use expen- Further, using the process and bath described in this invention, magnesium, non-compatible metal assemblies maybe treated after fabrication, if desired. The Dow #17 treatment is good for general corrosion, but has little effect on galvanic corrosion.

The following examples are set forth to illustrate, but not to limit, this invention.

EXAMPLE 1 Two sets of 4 x 5 x 0.25" panels of A2318 magnesium alloy having a 1 X 5 x 0.125" mild steel strap coupled to the center of the magnesium panel with three A" mild steel round head bolts were treated, respectively, with: the Dow #17 anodize, and the immersion process and bath described as preferred in Table I of this application. After treatment, both sets of panels were coated with a clear primer and subjected to corrosion tests, the results of which appear below in Table II.

1 Results tabulatedv as relative values, i.e., 10 showing no corrosion and 0 being percent failure of the general areas only.

Coating neutralized by immersion in 5 percent sodium bifiuoride for 1 minute at room temperature before application of paint primer.

As is shown by the table, the immersion process and solution described in this invention is superior over long exposure periods. The tests were run on magnesium-dissimilar metal assemblies, since these would show a more aggravated corrosion in a shorter time than would plain magneisum test panels.

EMMPLE 2 LEI-142 magnesium alloy panels having the nominal composition of 14 percent Li, 2 percent Al, and the balance Mg were used in a stagnant immersion test with 3 percent NaCl as described in AST M B43T. The panels were 3 x 6 x 0.100" with four x flat head 7 mild steel fasteners attached. These samples were not painted after treatment. Table III.

The results are shown below in Thus, under this test, it can be seen that the treatment described in this invention results in better corrosion resistance than the prior recognized methods.

It is to be noted, that a true tin coating is deposited on the dissimilar metal, while about a 50-50 mixture of Sn and MgSn(OH) is deposited on the magnesium surface.

We claim:

1. A process for providing a protective coating comprising tin on metal articles of the class of magnesium, magnesium base alloys, and assemblies thereof with metals of the class of iron, steel, copper, and brass, which comprises contacting the clean metal article with an aqueous bath comprising a water-soluble stannate in a concentration of from 37 to 60 grams per liter, a water-soluble pyrophosphate in a concentration of from 30 to 100 grams per liter, and a water-soluble caustic alkali in a concentration of from 7.5 to 12.5 grams per liter.

2. A process as in claim 1 wherein the water-soluble stannate is K SnO -3H O and the water-soluble pyrophosphate is Na P O 3. A process as in claim 1 wherein the water-soluble stannate is Na SnO 3H O.

4. A process as in claim 1 wherein the concentration of the water-soluble stannate is from 37 to 55 grams per liter and the concentration of the water-soluble pyrophosphate is from 30 to 55 grams per liter.

5. A process as in claim 1 wherein the caustic alkali is NaOH.

6. A process as in claim 1 wherein the caustic alkali is KOH.

7. A process as in claim 4 wherein from 7 to 30 grams per liter of a water-soluble acetate are added to the aqueous immersion coating bath.

8. A process as in claim 7 wherein the Water-soluble acetate is NaC H O 3H O.

9. A process as in claim 7 wherein the water-soluble acetate iS NH4C2H3O2.

10. A process as in claim 7 wherein the water-soluble acetate is KC H O 11. A process as in claim 4 wherein from 7.5 to 12.5

grams per liter of a water-soluble caustic alkali and from 7 to 30 grams per liter of a water-soluble acetate are added to the aqueous immersion coating bath.

12. A process as in claim 11 wherein the stannate is K SnO -3H O, the pyrophosphate is Na P O the caustic alkali is NaOH, and the acetate is NaC H O -3H O.

13. An aqueous immersion coating bath composition comprising from 37 to grams per liter of a water-soluble stannate, from 30 to grams per liter of a watersoluble pyrophosphate, and from 7.5 to 12.5 grams per. liter of a water-soluble caustic alkali.

14. An aqueous immersion coating bath composition as in claim 13 wherein the stannate is K SnO -3H O and the pyrophosphate is Na P O 15. An aqueous immersion coating bath composition as in claim 13 wherein the stannate is Na SnO -3H O.

16. An aqueous immersion coating bath composition as in claim 13 wherein the concentration of stannate is from 37 to 55 grams per liter and the concentration of pyrophosphate is from 30 to 55 grams per liter.

17. An aqueous immersion coating bath composition consisting essentially of a water-soluble stannate in a concentration of from 45 to 60 grams per liter and a Watersoluble pyrophosphate in a concentration of from 45 to 100 grams per liter.

18. An aqueous immersion coating bath composition as in claim 13 wherein the caustic alkali is NaOH.

19. An aqueous immersion coating bath composition as in claim 13 wherein the caustic alkali is KOH.

20. An aqueous immersion coating bath composition as in claim 13 wherein a water-soluble acetate is added to the bath in an amount to give a concentration of from 7 to 30 grams per liter.

21. An aqueous immersion coating bath composition as in claim 20 wherein the acetate is NaC H O -3H O.

22. An aqueous immersion coating bath composition as in claim 20 wherein the acetate is NH C H O 23. An aqueous immersion coating bath composition as in claim 20 wherein the acetate is KC H O 24. An aqueous immersion coating bath composition as in claim 20 wherein the stannate is K SnO '3H O, the pyrophosphate is Na P O the acetate is NaC H O and the caustic alkali is NaOH.

References Cited by the Examiner UNITED STATES PATENTS 6/1937 Oplinger 204-54.1 8/1960 Balden 117-130 Dedication 3,231,396.Herbert K. De Long and Charles TV. Glesner, Midland, and James A. Brown, Essexville, Mich. STAN N ATE IMMERSION COATING FOR MAGNESIUM, MAGNESIUM-DISSIMILAR METAL COUPLES, AND OTHER METALS. Patent dated Jan. 25, 1966. Dedication filed Aug. 9, 1974, by the assignee, The Dow Chemical Company.

Hereby dedicates to the Public the remaining term of said patent.

[Ofiicz'al Gazette March 11, 1.975.] 

1. A PROCESS FOR PROVIDING A PROTECTIVE COATING COMPRISING TIN ON METAL ARTICLES OF THE CLASS OF MAGNESIUM, MAGNESIUM BASE ALLOYS, AND ASSEMBLIES THEREOF WITH METALS OF THE CLASS OF IRON, STEEL, COPPER, AND BRASS, WHICH COMPRISES CONTACTING THE CLEAN METAL ARTICLE WITH AN AQUEOUS BATH COMPRISING A WATER-SOLUBLE STANNATE IN A CONCENTRATION OF FROM 37 TO 60 GRAMS PER LITER, A WATER-SOLUBLE PYROPHOSPHATE IN A CONCENTRATION OF FROM 30 TO 100 GRAMS PER LITER, AND A WATER-SOLUBLE CAUSTIC ALKALI IN A CONCENTRATION OF FROM 7.5 TO 12.5 GRAMS PER LITER. 